Add local test fixtures that were previously imported from
clan-core. This is necessary because clan-core's fixtures use
Path(__file__).parent to locate template files, which doesn't work
when clan-core is installed as a package from the nix store.
The local fixtures include:
- clan_flake: Creates test flakes with inventory configuration
- offline_template: Session-scoped template with pre-generated
flake.lock
- Supporting fixtures for git credential and GPG signing configuration
Also adds CLAN_CORE_PATH environment variable to the devShell, which
points to the clan-core flake input for tests that need to
substitute flake inputs.
collinarnett
ffda880708
